The present invention relates to a belt stretcher unit designed for assembly on a drive having a flexible member, e.g. a belt or chain, the unit being designed in particular for a motor vehicle, for maintaining drive and, at the same time, enabling tension adjustment of the flexible member.
Drives of the aforementioned type, designed in particular for transmitting drive from the drive shaft on an internal combustion engine to the accessories on the same, such as the alternator, water pump, power steering, air conditioner or the like, are known to employ a flexible member consisting of a continuous belt, usually made of rubber. The belt is wound partially round a number of pulleys, one connected to the drive shaft and the others to the aforementioned accessories for transmitting drive. The aforementioned drive also comprises a belt stretcher unit, the function of which is to ensure a given belt configuration and, at the same time, provide for tension adjustment on the belt.
The belt stretcher unit usually comprises a pin, pivoting round a first axis, and a pulley turning on the pin round a second axis eccentric with the first. The said pulley is supported on the pin by means of a rolling bearing or, quite simply, a ring of rolling bodies. The pin on the belt stretcher unit is usually secured to the engine block by a screw inserted in a hole on the pin the axis of which is the first axis already mentioned.
When the pin on such an arrangement is turned appropriately round the first axis, the said second axis round which the pulley turns is also shifted so as to set the drive belt to a given tension. When the tension slackens due to stretch on the belt during operation, initial tension on the belt may be reset by turning the pin round the first axis to a second position. Belt stretcher units of the aforementioned type involve a number of drawbacks.
First, resetting initial tension is frequently required, even after a fairly brief running period, owing to stretch during operation of the drive, caused in particular by creeping of the belt material.
Second, on drives of the aforementioned type, the belt is subjected to severe flexural vibration which, besides creating disturbing noise, may even displace the belt to the extent that it is released from its pulleys. These drawbacks are particularly noticeable on drives fitted with belts toothed on both faces or, as on currently-used engines, when rotation speed is particularly high. Furthermore, on drives between widely-spaced components supported on frames subject to severe thermal expansion during operation, the change in length produced by such thermal expansion may result in excessive load on the belt or a big reduction in shrinking tension.